Geology in space

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Winter in the northern hemisphere is synonymous with snow (at least in films and Victorian novels). The Earth is unusual because it has water can commonly be found on its surface in solid, liquid and gas form, and both solid and liquid water can rain/snow/sleet/drizzle from the sky. So what happens on other bodies in the solar system?

Clouds of ice crystals on Mars – Ready to snow? (NASA/JPL)

Most water on the surface of Mars is now trapped in its glaciers/ice caps, but what about snow? Whilst there is less water vapour in the Martian atmosphere than the Earth’s, Mars still has clouds made of ice crystals. These ice crystals can fall slowly out of the sky, but at night under the right conditions, snowstorms can occur. Most of this snow will vaporise before it hits the ground, and any that do make it to the ground will likely vaporise during the day.

Maxwell Montes the highest mountain on Venus, appears brighter than the surroundings because of sulphide frost (Image NASA/JPL)

Venus has the hottest surface temperature of any planet in the solar system. Water snow is not possible. The atmosphere of Venus contains lots of sulphur and temperatures hot enough to melt lead, it is so thick we can’t see the surface of the planet so we have collected radar data to understand what it looks like. One feature of this data is that on the tops of some high mountains there is an increase in the number of radio waves reflected, giving the appearance that the tops of mountains are brighter, just as snow on the top of mountains is brighter than the surrounding rock.

Whilst it is far too hot for snow on Venus, it is thought that these mountain tops are covered in a form of frost caused by metal sulphides, which forms only at high elevations and it’s this which makes is appear more reflective to radar.

Mercury, my main planet of study, doesn’t have snow-capped peaks, being too hot over the majority of the planet and no atmosphere. So the nearest feature to snow is ice in the craters close to the north and south poles, these craters are permanently in shadow protecting the ice from the boiling Sun. Without an atmosphere there isn’t any form of precipitation other than a daily micrometeorite shower.

The south pole of Mercury, the black areas are permanently in shadow and thought to contain water ice. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Titan, a moon of Saturn is much to far out from the Sun to have water snow, and any water near the surface, just like in the bleak midwinter, is like a stone. It does snow on Titan, near the poles hydrocarbons can crystallize out in the atmosphere and then fall as snow down onto the surface.

Another moon of Saturn, Enceladus, is probably the most visually spectacular example, geysers on the south pole can be seen erupting water vapour into space. Some of this water is lost in orbit and forming the E ring of Saturn, but some of the water falls back down onto the surface as ice crystals. As this fall back it is happening without an atmosphere it doesn’t really count as snow but is a great excuse to show the stunning photos of the plumes.

Venus is probably the only other planet still volcanically active other than Earth, it has lava flows. Venus express has shown sulphur dioxide changes and hot spots on the surface which suggest that it is still volcanically active.

Heat map of Idunn mons, showing a recent lava flow (ESA/Nasa/JPL)

The volcanoes on Venus should be similar to hotspot volcanoes on Earth like Hawaii. On Venus, they are mostly similar but on much larger scale.

Pancake domes on venus (Nasa/JPL)

Other volcanoes seen on the planet are called pancake volcanoes. 15 km diameter but only 1 km high these are an artefacts of higher viscosity lavas, which on Earth would build up to steep-walled stratovolcanos atmospheric pressures (90 times that of Earth’s atmosphere) prevents more viscous lavas from building up vertically and the high surface temperature of Venus which allows the lava to flow for longer before setting forming pancake shapes.

Sometimes these volcanic structures can undergo collapse along the edges of them with radiating valleys around the outside, creating the appearance on an insect with legs. these are known as scalloped margin domes or Tick – Like structures.

The Tick – a scalloped dome volcano on venus (NASA/JPL)

The volcanoes of Venus show what a difference that the conditions on a planet can make to the shape and style of surface features even if the processes seem similar processes to those on Earth.

Venus is our closest neighbour both in terms of distance and size. It is also a very alien world with surface temperatures of 46O °C and clouds of sulphuric acid.

To get an idea of what the surface of Venus looks like we have to use radar to see through the thick clouds and build up a topographic model, and what we see is a world with features which are in many ways similar to our own.

Mountain ranges, showing folded and faulted rocks structures surrounded by large flat plateaus, are very similar to the mountain chains on Earth. Rifts, which are cracks in the surface of the planet along which volcanoes generate new crust are found on both planets. Large volcanoes are found all over the surface.

However whilst the features seem similar on the surface, there distribution there are stark differences: on Earth, these are linked to plate tectonics, rifts form where plates move apart or break up. Mountains form as the crust crumples as these plates collide. There are also deep trenches such as the Mariana trench, linked to subduction as plate sinks down back into the mantle as a recycling of the surface. The only major features not caused by plate tectonics are hotspot volcanoes such as Hawaii. Even these hotspots, show the fingerprints of the movements of plates, which causes them to form chains of volcanoes as the plates move over the source creating lines of islands on the surface of the Earth.

Hawaii islands, formed by the movement of the plates over a hot-spot (JSC)

On Venus, the volcanoes do not form hotspot chains like those on Earth but instead appear at random all over the surface. Whilst both trenches and rifts are seen they are isolated and discontinuous. In short, just like with Mercury, it is a one plate world without plate tectonics.

To understand why this is and why Venus seems to have a universally young surface (even parts of Earth’s surface can be several billion years old) we can look at a side effect of plate tectonics: heat removal. the generation of crust at ridges allows heat to move to the surface and be emitted. Without this plate tectonics, heat builds up under the surface over time. If heat builds up over time, it is possible that a layer of hot rock builds up and the hard crust gets thinner as it warms and softens from the underneath and melt.

The Colorado River slowly cutting its way through rock over millions of years has formed a 450 km long, and 1.8 km deep canyon called the Grand Canyon. Whilst up close and even from space it looks impressive, in reality, it’s fairly small compared to other features which scar the planets across the solar system. Today, we’ll look at the three largest rifts and canyons in the solar system.

The Grand Canyon in the south if the image at seen from the ISS (NASA)

Valles Marineris on Mars at 4000 km long is 9 times as long as the Grand canyon and stretches 1/4 of the way around the entire planet. There are a few theories about how it formed, the most prominent is that the nearby Tharsis bulge (an area which hosts the tallest volcano in the solar system) caused tension in the crust, which lead to it tearing and pulling apart, in a process known as rifting. Erosion then would have deepened the valley further and has produced outflow channels at the end of it.

Valles Mariners, Mars (NASA)

One Venus, Baltis Vallis is even longer at nearly 7000 km long, its ends are covered so its original length is unknown. This feature, like the Grand Canyon, was also likely formed by erosion. Instead of water, high-temperature lava flows would have torn up, melted, and dissolved the surfaces as they flowed over them, slowly wearing them away in the same way rivers do on Earth. Similar features were probably once present on the early Earth, Komatiite lavas, found in Australia also show signs of eroding the rocks beneath them and forming channels as they flow through an area.

a 600km segment of the 7000 km Balits Valley from Radar data collected by Magellan, the channel can be seen as a meandering line going diagonally across the image arrow to arrow (NASA/JPL_

However, the largest in the solar system is right here on Earth the 10’000 km long and up to 8.5 km deep Atlantic Ocean is a large cut into the Earths Surface. Like Valles Marineris, this was formed by rifting rather than a valley. Tectonic forces pulling and pushing Europe and Africa away from the Americas. Plate tectonics allowed the rift to develop much further and volcanism generated new oceanic crust was generated in between the two.

Bathymetric image of the Atlantic ocean (NOAA)

This quick overview of the largest valleys and rifts in the solar system highlights two main processes which can form them; erosion and tectonics. Volcanism on Venus and Mars, extensional forces tearing the crusts show these planets have been tectonically active. In the next few posts I’ll examine tectonics work across the solar system.

Happy Solstice everyone! Today in the northern hemisphere is the shortest day (longest in the southern hemisphere).

If you imagine a stick through the Earth around which the planet spins on a daily basis this is the Earth’s axis. If this axis was vertical, then the lengths of day would not change, throughout the year, however it actually lies out at an angle of 23 degrees from the vertical. At different points in its orbit the north will be pointing either in the direction of the sun or further away. Today the northern hemisphere is pointing directly away from the sun. This means that in the northern hemisphere the days are much shorter and the nights longer due to spending more time facing away from the Sun than towards it. In addition due to the curvature of the Earth the beams of light hitting the surface is more spread out towards the poles than the equator and so the amount of incoming energy spreads out, these two mean that the climate gets colder during the winter. What about other planets, are there seasons and how are they manifested?

The north pole of Mercury, some of the craters are permanently in shadow (NASA/ John Hopkins/Carnegie Institution)

There are huge temperature variations on Mercury related to the eccentricity of its orbit (how elliptical it is rather than circular) linked with a 3:2 ratio of years to days but these do not cause temperature changes in latitude.

Venus has a tilt of 177 degrees, what this means is that it is completely flipped over when you look at its rotation (it spins clockwise whilst the other planets spin anticlockwise)

That being said it means that the axis is only about 3 degrees off the vertical and with the very efficient heat transport in the dense atmosphere the temperature is fairly constant over the whole globe and that Venus doesn’t have a strong seasonal changes.

With a similar axial tilt to Earth (25 degrees) Mars also has seasons, which are about twice as long as on Earth (due to the longer year). This leads to growing and shrinking of carbon dioxide ice caps and temperature changes just as on Earth. Intriguingly images from the Mars reconnaissance orbiter have shown linear features called recurring slope lineae forming on crater edges, these features grow during the warmest months then disappear during the coolest month. They are thought to be formed from brines (very salty water) which melt and run down the slope, they do not appear in the winters due to it being too cold for these brines to melt, although the source of the water is not currently certain.

Saturn has seasons which last around 7 Earth years, changes in cloud composition and occurs during this transition and there are increased storms during spring

Uranus is lying on it side, meaning that the axial tilt is just of the equator which means that the poles experience 42 years of day light followed by 42 of darkness, the change in temperature between the side facing the sun and the side facing away the sun probably has an effect on its climate however as it has only been briefly visited by the Voyager 2 probe little is known about the long term seasonality ice giant.

Finally Neptune has a similar axial tilt to Earth of 28 degrees, at the moment a lack of observational evidence makes it difficult to say if it has any strong seasonal effects although an increase in cloud cover has been noticed by Hubble as it transitions into a 40 year long summer.